Cast coil transformers have windings that are solidly-cast in resin under a vacuum in a mold. This manufacturing process covers the transformer windings in a strong, low-viscosity, epoxy resin with a high dielectric strength. It also protects the device from harsh or severe operating environments with high electrical demands. Cast coil transformers are well-suited for applications that require long service life and little maintenance. They represent a cost-effective alternative to medium-voltage, liquid-filled transformers that may pose environmental concerns and incur increased insurance costs. Specifications, materials of construction, application requirements, and certifications are important parameters to consider when these electrical and electronic components. Cast coil transformers also differ in terms of enclosure type, and provide features such as forced-air cooling.

Product specifications for cast coil transformers include primary voltages, secondary voltages, basic impulse level (BIL), and temperature rise. The primary voltage is applied to the terminals of the primary winding, the part of the transformer that is connected to the energy source. With cast coil transformers, primary-voltage values may range from 600 V to 35 kV. The secondary voltage is the coil winding supplying the output voltage, which is measured at the output terminals of the power supply. BIL ratings are indicators of dielectric strength. As a rule, cast coil transformers with a better BIL rating provide greater dielectric strength, an important consideration for withstanding overvoltages and electrical and thermal stress over time. Finally, temperature rise is the temperature increase over ambient due to load.

Cast coil transformers contain special epoxies and core materials. To withstand harsh environments, epoxy resins should use filter materials that impart better arc resistance, higher heat conductivity, and improved temperature ratings. Examples of these filter materials include glass-fiber mat and wollastonite fibers. Often, the cores of cast coil transformers consist of thin, individually-insulated, high-permeability, grain-oriented, non-aging, silicone steel. This rust-resistant core steel is laser-treated and available in grades such as M2, M4, and M6. Typically, the cores of cast coil transformers are held in place with steel straps and both vertical and horizontal braces. Organizations such as Underwriters Laboratories (UL) and the Canadian Standards Association (CSA) provide ratings for both the epoxy resins and the core materials.